1. Technical Field
The present disclosure relates generally to a buck converter, and more particularly to a buck converter with a variable-gain feedback circuit for transient responses optimization.
2. Description of Related Art
Switching buck converters are widely used as voltage regulators in battery-powered portable devices to achieve high power-conversion efficiency. FIG. 10 shows a block diagram of a conventional buck converter which includes a power stage and a controller. Also, FIG. 11A and FIG. 11B are schematic circuit diagrams respectively showing operations of a high-side ON state and a low-side ON state in FIG. 10. The synchronous topology of the buck converter has two switches, one is referred to as the high-side switch SH and the other is referred to as the low-side switch SL.
The controller mainly has an error amplifier with compensation components 21, a pulse width modulation (PWM) generator 22, and a driver 23. The error amplifier with compensation components 21 receives a reference voltage VREF and a feedback voltage VFB to generate an error voltage. The feedback voltage VFB is obtained by dividing the output voltage VO by resistors RO1,RO2. The PWM generator 22 receives the error voltage and a voltage, which may be a sawtooth signal, a sensed inductor current IL or the output voltage VO, to generate a PWM signal to control the high-side switch SH and the low-side switch SL. Accordingly, in steady state conditions, this cycle of turning the high-side and low-side switches SH,SL ON and OFF complimentary to each other regulates the output voltage VO to its targeted value.
As shown in FIGS. 12A-12B, the inductor current IL is increased when the high-side switch SH is turned on; on the contrary, the inductor current IL is decreased when the low-side switch SL is turned on.
To integrate more multi-functional system-on-chip (SOC) applications into battery-powered portable devices, buck converters should provide a regulated voltage despite large and frequently varying load current. In addition, to reduce the power consumption of digital systems, buck converters should also offer an adjustable output voltage for realizing dynamic voltage scaling (DVS). However, if the transient responses of the buck converters are slow, a large output voltage undershoot/overshoot or a long settling time will occur, resulting in reduced reliability of the systems or degraded signal-to-noise ratio (SNR) performance of noise-sensitive circuits. These problems can be alleviated by a buck converter that simultaneously achieves both fast load transient response and fast DVS transient response.
To improve the power stage for achieving fast transient responses, asynchronous low dropout regulator (LDO) or a power switch can be connected in parallel with the buck converter to bypass the output inductor LO and prevent the transient response from being limited by the charging slope of the output inductor current IL. However, the chip area and power consumption will increase.
To improve the controller for achieving fast load transient response, a feed-forward path in V2-based controls can deliver the output voltage directly to the PWM generator 22 and bypass the error amplifier with compensation components 21, which limits transient responses. The adaptive pole-zero position technique can reconfigure the error amplifier with compensation components 21 to extend the loop bandwidth during load transient. In addition, for achieving fast DVS transient response, the end-point prediction (EPP) technique can predict the output voltage of the error amplifier with compensation components 21 to accelerate the transient response. However, the aforementioned techniques for improving the controller can only achieve either fast load transient or fast DVS transient response.